As a keyboard used for a personal computer and the like, a capacitive keyboard including a plurality of capacitive keys has been proposed and put into practical use. A capacitive keyboard includes a plurality of drive lines and a plurality of sensing lines that intersect the drive lines, and capacitive keys are arranged at the respective intersections of the drive lines and the sensing lines. Upon any one of the plurality of keys being depressed, the capacitance between two electrodes of the key is increased, and accordingly, current flows from a corresponding one of the drive lines through the depressed key to a corresponding one of the sensing lines. By detecting the current, the depressed key can be recognized (see, for example, PTL 1).
Now, a capacitive keyboard of the related art will be described below with reference to FIGS. 10 and 11. FIG. 10 illustrates the arrangement configuration of the capacitive keyboard of the related art, and FIG. 11 is an equivalent circuit diagram of the capacitive keyboard of the related art. As illustrated in FIG. 10, in the capacitive keyboard of the related art, a plurality of drive lines M (M-1, M-2, M-3 . . . ) and a plurality of sensing lines N (N-1, N-2, N-3 . . . ) are arranged to intersect one another. Each of the drive lines M is connected to a drive circuit 101, and each of the sensing lines N is connected to a sensing circuit 102.
Keys 103 (103a, 103b, and the like) are arranged at the respective intersections of the drive lines M and the sensing lines N. Upon any one of the keys 103 being depressed, the capacitance at the intersection of a corresponding one of the drive lines M and a corresponding one of the sensing lines N can be changed. Specifically, upon any one of the keys 103 being depressed, the capacitance is increased. Thus, keys are expressed by symbols of variable capacitors in the drawings.
The drive circuit 101 alternatively applies an H-level voltage to each of the drive lines M over a fixed period. Accordingly, for example, upon the key 103a illustrated in FIG. 10 being depressed, if an H-level is set in the drive line M-4, current flows from the drive line M-4 through the sensing line N-5 to the sensing circuit 102. That is, current flows through a path indicated by arrows Y0 and Y1 in FIG. 10. Accordingly, while the drive line M-4 is set at the H-level by the drive circuit 101, on the basis of detection of the voltage of the sensing line N-5, the sensing circuit 102 can detect that the key 103a has been depressed.
However, if a key other than the key 103a is depressed as a result of erroneously or intentionally depressing a plurality of keys, for example, the sensing circuit 102 cannot detect the exact voltage in some cases. For example, while the key 103a is depressed, if the key 103b is further depressed, current that flows in the drive line M-4 is supplied through the sensing line N-5 to the sensing circuit 102 and through the drive line M-6 to the ground. That is, current flows through a path indicated by an arrow Y2 in FIG. 10.
This phenomenon will be described with reference to the equivalent circuit illustrated in FIG. 11. If only the key 103a is depressed, current that flows through a capacitor C101 flows via a resistor R2 to the ground. Thus, by measuring the voltage at a node P1 of the resistor R2, it is possible to detect that the key 103a has been depressed. At this time, if the key 103b is further depressed so that current flows through a capacitor C102, the voltage detected by the sensing circuit 102 is lower than the voltage detected during a normal operation. Accordingly, the voltage might not reach a threshold for determining whether or not the key has been depressed, and the detection might fail. If three or more keys are depressed at the same time, such detection failure may occur with a higher possibility.
Recently, there has been a demand for proposing a keyboard having a function of, in addition to detecting whether or not a key has been depressed, detecting the depressed amount of a key (stroke of a key). In addition, there is a demand for a keyboard to have a function that enables various input operations by depressing a number of keys at the same time. Although the technique disclosed in PTL 1 can detect whether or not a key has been depressed, it is difficult to detect the depressed amount of the key. In addition, if a plurality of keys are depressed at the same time, a problem arises in that the depressed amount of each key cannot be detected with high accuracy.